Energieeffizientere Konstruktionen insbesondere in der Automobilindustrie und bei
deren Zulieferern erfordern die Substitution schwerer Bauteile aus Stahl und anderen
metallischen Werkstoffen durch entsprechende Leichtbauvarianten aus Kunst- bzw.
Verbundwerkstoffen. Dieser Trend setzt sich nach der erfolgreichen Einführung von
Kunststoffbauteilen im Innenraum von Automobilen auch vermehrt bei sicherheitsrelevanten
Konstruktionen z.B. im Motorraum durch. Um die hohen Anforderungen an
die mechanischen Eigenschaften der verwendeten Materialien erfüllen zu können,
werden überwiegend Faserverbundwerkstoffe eingesetzt. Da im Rahmen der Bauteilentwicklung
der Einsatz der Finite-Elemente-Methode (FEM) mittlerweile zum Stand
der Technik gehört, müssen auch für Faserverbundwerkstoffe die entsprechenden
Materialmodelle auf ihre Anwendbarkeit hin überprüft und gegebenenfalls weiter oder
neu entwickelt werden.
In dieser Arbeit wird ein Versuchs- und Auswertekonzept zur Bestimmung der mechanischen
Materialkennwerte von langglasfaserverstärktem Polypropylen (PP-LGF)
vorgestellt und validiert. Es wird orthotrop visko-elasto-plastisches Materialverhalten
des Verbundes angenommen. Zur Ermittlung der Datensätze für die FE-Simulation
werden Zug- und Schubversuche bei fünf unterschiedlichen Abzugsgeschwindigkeiten
von quasistatisch bis 10 m/s durchgeführt. Dabei werden mit Hilfe der Grauwertkorrelationsanalyse
berührungslos Dehnungsfelder auf der Oberfläche der Probekörper
erfasst und später mit Kraft-Zeit-Daten zu Spannungs-Dehnungs-Kurven verrechnet.
Das orthotrope Materialverhalten von PP-LGF wird berücksichtigt, indem
sowohl Zugversuche an Probekörpern mit vorwiegend in Zugrichtung als auch an
solchen mit überwiegend quer dazu orientierten Fasern ausgewertet werden.
Die Dehnratenabhängigkeit des Materials wird über einen visko-elasto-plastischen
Ansatz in 1D getrennt für zwei Zugbelastungsrichtungen mathematisch beschrieben
und die Parameter über einen Least-Square-Fit unter Verwendung des Levenberg-
Marquardt-Verfahrens bestimmt. Im Rahmen eines Vergleichs experimentell ermittelter
Verschiebungs- und Dehnungsfelder einer gelochten Zugprobe mit den Ergebnissen einer korrespondierenden FE-Simulation wird ein orthotrop elasto-plastischer
Simulationsansatz in 3D validiert. Dabei wird eine Formulierung nach HILL für orthotropes
Fließen berücksichtigt. Am Ende der Arbeit wird gezeigt, inwieweit das erfolgreich
validierte Modell auf eine komplexere Bauteilgeometrie übertragen werden
kann. Es wird deutlich, dass bei sehr komplexen Geometrien die Qualität der Simulationsergebnisse
nicht nur vom verwendeten Materialmodell und der Güte der Materialparameter
abhängt, sondern zunehmend von der Qualität einer der FEM vorgeschalteten
Füllanalyse.
The scarcity of raw materials such as oil and natural gas has led the necessity to design
more and more energy-efficient constructions. This applies particularly to the
automotive industry and its component suppliers. There are two possibilities to meet
this obligation. On the one hand, one can develop new forms of drive concepts, e.g.
hybrid or electric motors. On the other hand, the reduction of weight can significantly
diminish the consumption of fuel. Especially in the aerospace industry every gram
less counts. To put this requirement into action traditional materials, e.g. steel and
cast iron, are replaced by new lightweight materials. This can be miscellaneous light
alloys or one of several fibre reinforced plastics. After the production of plastic devices
such as display panels in cars, more and more security-relevant plastic devices
appeared.
This report deals with one important material within the group of glass fibre reinforced
thermoplastics i.e. long-glass-fibre reinforced polypropylene. Polypropylene with approximately
14% of the worldwide usage of plastics is very cheap and can support
companies in cost-saving. This aspect becomes more and more important considering
the expansion of global business competition, especially in the automotive industry.
With approximately 50%, glass-fibre reinforced polypropylene is the dominant
material in the group of glass fibre reinforced thermoplastics. To counteract the demand
of shorter development times in the design process the automotive industry
and its suppliers count on simulation tools such as the finite-element (FE)-simulation.
For this reason the quality of the mathematical material models has to be improved
and its parameters have to be identified. After choosing a suitable material model and
identifying the corresponding parameters, a verification of both has to follow to assure
their reliability. This is typically carried out by comparing measurement with simulation
data at distinctive points of the considered geometry. With advanced performance
of computer hardware and measurement systems it is possible to measure
specimens and more complex geometries at high strain rates by means of high
speed cameras and to conduct FE-Simulations in a justifiable time with sufficient accuracy Thus the following chapter firstly discusses the basics of continuum mechanics with
its well-established mathematical formulations for anisotropic elasticity, plasticity and
viscoelasticity and the corresponding rheological models. In chapter three the properties
of the matrix material polypropylene are presented with more precise consideration
of the strength and failure of the composite material under tensile load depending
on the fibre length and volume. Thereafter, the geometry and dimensions of the
specimens for tensile and shear tests are presented, together with a test sample
which is used as an example for the design of automotive structures via coactions of
measuring and simulation techniques. Chapter four deals with the state of the art in
high speed testing, the experimental setup and its mode of operation as well as the
explanation of different kinds of evaluation software which is used in this work.
Here, the main focus is aimed at the determination and interpretation of displacement
and strain fields with the grey scale method. The following paragraph describes different
initial examinations of specimens and the test sample. In this context, exposures
of fracture surfaces and polished micrograph sections were examined by
means of optical and scanning electron microscopy. In this way discontinuities and
cracks caused by the injection moulding process are detected. The quality of the
specimens and the test sample and their suitability for systematic material testing are
finally evaluated by means of thermoelastic stress analysis and a laser extensometer.
With these systems a direct optical strain measurement can be accomplished to
screen the homogeneity of specimens and parts.
In chapter six an approach for an experimental method is presented for measuring
displacement fields of the surface of specimens of long-glass-fibre reinforced polypropylene
at high strain rate. A strategy is introduced for reasonable data evaluation
to characterise stress strain behaviour under tensile and shear load. Specimens of
long-glass-fibre polypropylene with a fibre volume content of 30% and 40% respectively,
are tested and analysed. Anisotropy of the material is considered by testing
specimens with fibres oriented either in the direction of the tensile load or perpendicular
to it. The resulting stress strain curves for the two main directions are used as
basis for identifying material properties for an orthotropic constitutive law. Hence the
elastic parameters that are collected from stress strain curves are the axial modulus
in fibre direction and perpendicular to it, the shear modulus in the fibre plane and the
matrix plane and finally the lateral contraction in fibre direction, at right angle to it and
in the matrix plane. After having collected the parameters for elasticity, the HILLcriterion
is utilised to describe orthotropic plasticity. The different stresses at the
yielding points related to the designated directions are used to compute the parameters
of the HILL-criterion. Finally, in order to describe the strain-rate dependent behaviour
of the material, a one dimensional rheological model with four relaxation
terms is utilized to represent the set of curves resulting from tests of specimens at five different strain rates. The viscoelastic parameters are identified by means of a
least square approach using the Levenberg-Marquardt algorithm.
In chapter eight, the elastic-plastic orthotropic material model is verified for low strain
rates. A specimen with a hole in its middle is exposed to a tensile load until break and
simultaneously measured by means of a CCD-camera to obtain the two-dimensional
displacement field on its surface. Comparison of the experimental displacements with
the displacements of a finite-element model at the same points shows the quality of
the material model and its parameters. Finally, in chapter nine, the results of a filling
simulation of the test sample is shown and again a comparison of measured and
simulated data is presented. This we describe the potential and the limits of current
filling-simulation-software in conjunction with a popular finite-element-tool like
ABAQUS. To conclude appropriate mathematical characterisation and reliable parameters
of long-glass-fibre reinforced polypropylene require stringent experimental
and theoretical characterisation. Complete specification of such a complex material is still very time-consuming and prone to mistakes during the whole sequence of operations.
For this reason it is very important to improve the reliability of every single step
in product engineering. Hence the performance of a mathematical material model is
not only dependent on its formulation but also and particularly on the quality of its
material parameters whether they are determined directly from material tests or by
means of special optimization software. The best optimization tools can only optimize
material parameters reliably if the underlying experimental data is just as reliable.